Microphone amplifier



ug. 27, 1940. s, SEARS 292212,84@

MICROPHONE AMPLIFIER Filed July 16, 1936 Patented Aug. 27, 1940 UNITED STATES PATENT OFFICE IVIICROPHONE AMPLIFIER Application July 16, 193s, serial No. 90,860l

2 Claims.

This invention relates to amplifiers and has particular reference to so-called microphone amplifiers or boosters especially adapted for amplifying the voice currents of a portable hearing aid o device.

Heretofore, microphone amplifiers for portable hearing aid devices having the usual clamped diaphragm had a limited output in both energy and tone range, due. largely to poor diaphragm l damping, which introduced distortion of a large and detrimental value, These former devices also had undesirable frequency discrimination within the usable tone range and were difcult to manufacture, so much so that unless extreme care and precision were exercised in their manufacture, the response characteristics of apparently identical devices would vary widely.

In. accordance with the present invention, a microphone type of amplifier especially adapted for hearing aid devices vis provided, which is simple in construction, has an extended tone range, acts as though damped without corresponding loss of output eiiciency, has a high amplification and output, and can be manufactured in large quantities 'with excellent uniformity. f

The invention comprises a casing of small size, preferably adapted to be carried by and directly electrically connected to the usual battery packet, and containing the carbon granule microphone cell electrically connected `to the airor bone-conduction receiver, the carbon granules being in operative electromechanical contactv with the `specially-mounted diaphragm of an electromagnetic actuating means, actuated by the microphone or transmitter, for mechanically rearranging the energized mass of carbon granules to continuously change its conductivity in accordance with the iiuctuations in the vvoice currents flowing through the actuating means, whereby these currents are amplied in the wellknown manner of operation of carbon granule microphone amplifiers.

The diaphragm, loaded by the attraction of the electromagnetic actuating means, is distorted from its normal free shape jointly by this attraction and by the clamping means therefor, the latter positively dishing the diaphragm in a direction opposite to the direction of attraction of the electromagnetic actuating means, While 50 the former opposes this distortion by flexing the center of the diaphragm in the said direction, so that the diaphragm is doubly curved in crosssection. This distortion of the diaphragm is preferably obtained by properly tapering the 55 seat upon which the diaphragm is clamped at its periphery,r although this distortion may be procured in other ways, such as by forming the clamping ring seat eccentrically with one side lower axially than the other, or by pre-forming the diaphragm to impart a slight dish or crown thereto.

It will be seen that with this arrangement, the amplifier does not depend largely for its response characteristics upon the inherent iiexibility or stiifness of the material of the diaphragm, but 10` by virtue of this double distortion, different radial points thereon are placed under different internal tension, affording a degree of restrained exibility which enables it to `vibrate freely at various frequencies and act as though damped, 15'" procuring special operating characteristics without output loss. Among these special characteristics is the ability of the diaphragm to change its mass-stiffness ratio at different frequencies, so that it tends to resonate at various frequen- 20 cies, with the result that the amplier responds to a broad range of frequencies with uniform results. Also, with this arrangement, the diaphragm acts as though damped at its primary resonance point with little absorption of energy. 25 This effect allows closer adjustment of the air gap to obtain high output sensitivity and efficiency without liability of Contact between the diaphragm and the pole faces.

For a more complete understanding of the in- 30 vention, reference may be had to the accompanying drawing, in which:

, Fig. 1 is a side View of the microphone amplier of this invention with the cover plate of the enclosing casing removed;

-35 Fig. 2 is an enlarged axial section through the amplifier unit as seen along the line 2--2 of Fig. l;

Fig. 3 is a greatly enlarged and exaggerated view of the novel diaphragm arrangement; 0,

Fig. 4 illustrates a modified form of the dia- 4 phragm arrangement; and

Fig. 5 is a conventional Wiring diaphragm'for a hearing aid device.

Referring to Fig. l of the drawing, numeral ID designates a casing of hard rubber, phenolic resin, or the like, carrying contact prongs H which fit into corresponding sockets in a conventional battery packet I2, which is usually connected as is indicated in the circuit diagram of Fig. 5. The several parts of the hearing aid device include the transmitter-microphone I3, an airor bone-conduction receiver I4, and the microphone amplifier unit l5, having the Windings I6 and the microphone Il. Other circuit 55,

arrangements may be` utilized according to requirements.

The microphone amplifier unit I5, shown in enlarged section in Fig. 2, includes a tubular housing I8 of conducting materil, such as brass, which is closed at one end by a'non-conducting disc I9 of porcelain, phenolic resin', or the like, secured in place `by screws 20 kand carrying the carbon button ;21I electricallyconnected t0 the external contact 22.V` This contact "22 connects the microphone I7 in the circuit of receiver I4 by suitable connections with the casing Ill.

A clamping ring 23 threaded into the open opposite end of housing I8 cooperates with `the seat 24 in housing I8 to clampthediaphragm 25 in position. A ring of cotton Y'wadding'ZI interposed between diaphragm 25 and disc I 9 forms with the latter the carbon granulerchamber I 'I containing the mass of carbon particles 21 of suitable dimensions, either granular in form or comprising carbon balls..l l f Threadedinto thehousingfI-is a nonconducting plug v28 of hard Arubber, phenolic resin or the like, having secured to itsouter face by means of clampl'a bar magnet 30 contacting the poles of a pair of windings I6. The faces 3l of the poles lie incloselpromixity to the armature 32 of rdiaphragm, 25.forming an air gap which is adjustable by, screwing plug 28 into or out of housing ,I8.; The terminalsof the windings I6 are suitably connected in the hearing aid circuit by means ,of contacts 33 connected Yby leads 34 to binding .posts 35 on casing Ill, as is indicatedinFig. 1. y

As shownrin exaggerated form inFig. 3,.the seat 24 of housing I8Yis chamfered or beveled and the cooperating surfaceof clamping. ring 23 mayV be at or similarlyv chamfered or beveled. The angle A` atwhichthe seat; 24 is charnfered or beveled depends upon` the degree of distortion which is to be imparted thereby to the normally plane 4diaphragm Y25,v but. for.A purposes 'of .hearing aid devices the angle A vranges preferably between five minutesandone degree. Accordingly, when the plane diaphragm 25, which is preferably made -ofphosphnr vbronzebr some other suitablematerial, is :clamped between seat 24 and clamping ring 23,*the taperv or chamfer of the seat impartsa dish shape to the diaphragm 25,which is oppositetothe distortionsproduced by .the magnetic lload .at-the center of the diaphragm by the attractive force of the windings I6 energized by :the battery `I2 `and/or magnet 30. sinuous shape in cross section, asis indicated in exaggerated form in Fig. 3, 'causing diierent,- radial portions. thereof :ato: be under different de-. grees of tension in accordance with the doubleV distortion, eifected jointly bythe taper of seat 24 and the magnetic load applied to its "centerportion bythe .windings I5.; .-It will Vbe observedthat the diaphragm is supported onlyat its periphery.

' As is illustrated in exaggerated form in Fig. 4 the distortion of the diaphragm may also be obtained by `formingvthefse'at 24 AYeccentrically in such a way that; onev side :is axially Ilower by a small amountB than the opposite side, i. e., the plane of the vseat 24 is Lnot normal to the axis of the threads in the housing:`

The portion of the seat at Vthe right of Figure 4A is parallel tobut :spaced lbelow the lef-t `hand portion of the s'eat.-L j Linesdrawnradially or transverselyof the seat 24? are substantially perpendicular to the axis of the threads inthe housing. Thus, .whenthelidiaphragm is clamped flatly The diaphragm/.25;` accordingly .acquires aA against al1 portions of theeseat, itis distorted into a sinuous cross-sectional shape, for the reason that the opposite portions of the seat do not lie in a common plane. The clamping ring 23' is not similarly eccentric so that when it engages the diaphragm 25', the latter is distorted in a manner somewhat similar to that produced with the arrangement illustrated in Fig.,3, that is to say, the diaphragm is forced 'out-,of-its normal plane condition into a distorted shape which is generally somewhat sinuous, as is indicated in exaggerated form in Fig. 4. With this arrangement Vdifferent radial portions of the diaphragm are placed under different degrees of tension so that the required results are obtained. Other ways of obtaining this mechanical distortion of the diaphragm may be provided.

Instead of distorting a plane diaphragm in the manner described, the primary distortion can be provided by pre-forming the diaphragm with a slight dish or crown, which, afterrriounting in the amplifier, is modified by the magnetic1 attraction into the aforementioned doubly-curved-v shape. f .i

In operation, the voice currents impressed on the windings I6k of the microphone amplier of this invention result in vibration of the diaphragm to rearrangethe mass of carbon particles 2l to change its conductivity and effect amplication in the well known way. strained flexibility of the diaphragmfproduces an improved response curve due to its double curvature providing diiferent tension at different. radial points, which causes it to change its mass-V stiiiness ratio at different frequenciesA so that it tends to resonate at various frequencies. Also,

with this simple arrangement, all .diaphragms made according to Yproduction requirementshave uniform response characteristics, so that :microphone amplifiers embodying these/diaphragms may be made in large numbers with uniform` SUCCESS.

WhileV a preferred embodiment offthe inven--lf` tion hasV been illustrated and described herein, it is to be understood that the invention is not limited thereby but is susceptible of? changes in form and detail.- within its scope.

scribed as.. applied to microphone amplifiers, .it may be used with equalfacility in microphones,

, transmitters, receivers, loud-speakers and Vother.

acoustical instruments of all kinds iwithoutlrequiring .substantial bility, and the like. Itisalsonto `be understood that the term diaphragm employed hereinand* in the appended claims is not` necessarily con fined to circular diaphragms except as described,

strip diaphragms -or reeds.

shoulder having'an inner edge portion lyingin a 'plane inclined with respect to an axis, said axis being substantially perpendicular' to all lines which are perpendicular to saidl edge andt'olines tangent to saidvedge and lying-in said surface; means for clamping said edge portionsiiatlyn against vsaidy seat, wherebyethe diaphragm is disev torted into a r-wavy'- cross-sectional shape; and

The.; re-

modification..` Foril dia'-4 phragms of .relatively larger diameters than 1%. inch, which is the nominal diameter ofthe dial phragm of a portable hearing aid amplifier, the angle A varies in accordance with diameter viiex-il a surface forming alseatv` for the edge portionsv of thegsaid diaphragm,'said" sol Also, whiley the novel diaphragm arrangement has been de an electromagnet associated with said diaphragm for distorting its center portion into a doubly curved shape with the convex surface thereof directed toward said electromagnet.

2. In an acoustic device, the combination of a circular diaphragm, a housing, an annular inwardly projecting shoulder therein having a surface forming a seat for the edge portions of said diaphragm, said shoulder having an inner edge portion lying in a plane inclined with respect to an axis, said axis being substantially perpendicular to all lines extending radially from said axis and lying in said surface, means for clamping said edge portions iiatly against said seat, whereby the diaphragm is distorted into a wavy cross-sectional shape, and an electromagnet associated with said diaphragm for distorting its center portion into a doubly curved shape with the convex surface thereof directed toward said electromagnet.

STERLING G. SEARS. 

